The present invention relates to an underframe which serves as the framework of the body of a motor vehicle and, more particularly, to an underframe of a motor vehicle which includes side sills.
While the body of a motor vehicle is in many cases provided with a monocoque structure, the mechanical strength and rigidity of, among others, a large-size motor vehicle often relies on the underframe. Generally, such an underframe is made up of front and rear side frames respectively positioned in a front and a rear part of a vehicle body, and floor frames associated with that part of the floor of a passenger compartment which intervenes between the front and rear side frames. In the case of a passenger car, the underframe is additionally provided with side sills at opposite sides of the floor of the passenger compartment. To insure the rigidity of a vehicle body, the various frames and the side sills have to be firmly connected to each other.
The various members constituting the underframe of a passenger car as stated above each plays the role of a strength member of the vehicle body and therefore usually has a channel configuration. Such channel members cooperate with vehicle body panels which are connected thereto to define hollow closed sections. In a prior art underframe, the front side frames, for example, are provided with a side-open channel configuration and a front side panel is connected to the open side of each front side frame to define a closed section. The side sills are each implemented by a pair of side-open channel-like panels which are connected face-to-face to each other to define a closed section. Further, the floor frames are configured as a top-open channel members and cooperate with a floor pannel which is connected to their upper surfaces to define a closed section.
In a passenger car, a dashboard is provided to isolate an engine compartment side where the front side frames are disposed from a passenger compartment side where the side sills are disposed. The dashboard has also to be connected to the front side frames, floor frames and side sills. It is impossible to connect the dashboard to the upper surfaces of the side-open front side frames and those of the side-open side sills because flanges protrude upward from such frames. Usually, therefore, the front side frames, floor frames and side sills are interconnected by a front outrigger which extends in the lateral direction of the vehicle body, and the dashboard is connected to the upper surface of the outrigger. The outrigger is provided with a top-open channel configuration.
A problem with such a prior art implementation is that the side-open channel members such as the front side frames and side sills and the top-open channel members such as the front outrigger cannot be interconnected except at their side surfaces. In this condition, a load acting on one of associated member is transferred to only one side surface of the other member, resulting in poor transmission of load. This in turn lowers the efficiency of the underframe as a framework.
In light of the above, Japanese Laid-Open Patent Publication (Kokai) No. 57-140281 proposes a front outrigger with a twisted tubular configuration for interconnecting a front side frame and a side sill. Such a front outrigger successfully allows the closed section of the front side frame and that of the side sill to communicate with each other to thereby increase the strength of the framework. However, producing a twisted tubular outrigger by press-forming is extremely difficult and therefore requires disproportionate cost.
Further, the floor frame has hitherto been connected to the underside of a major part of a floor panel. Therefore, the dimension of the floor frame as measured in the up-down direction cannot be increased beyond a certain limit which is dependent upon the ground clearance, limiting the rigidity available against bending forces.